Most (all?) user-agents don't set the TE header (which specifies the transfer encodings the UA is willing to accept), so I doubt they would bother to decode unsolicited "Transfer-Encoding: gzip".
The Accept-Encoding header is used to specify the acceptable transfer encodings. Certainly all browsers set it, and most "advanced" frameworks will at least handle the common values of Transfer-Encoding regardless; a "malicious" crawler will almost certainly have to, as plenty of sites use Accept-Encoding along with User-Agent to block undesirable bots.
